1. Field of the Invention
The present invention relates to a magnetic core, more particularly to a saturable core which comprises a wound amorphous alloy sheet. The present invention also relates to a method for manufacturing such a magnetic core.
2. Description of the Prior Art
It is known to use a saturable core in a particle accelerator, a magnetic modulator for a low-impedance discharge laser, a power switch, a pulse generator, and the like.
FIGS. 1 and 2 illustrate an equivalent circuit of a saturable core used for a particle accelerator and the hysteresis loop of the saturable core, respectively. In FIG. 1, L.sub.1, L.sub.2, ------ L.sub.n denote saturable inductors. C.sub.1, C.sub.2, ------ C.sub.n denote capacitors which have capacitances equal to each other. The inductance of inductors L.sub.1, L.sub.2, ------, L.sub.n is at a high stage of the LC circuit. Direct current is applied to the input of the equivalent circuit and is first loaded in the capacitor C.sub.1. When the capacitor C.sub.1 is loaded and the saturable inductor L.sub.1 is saturated, the impedance of the saturable inductor L.sub.1 is decreased, with the result that the electric charge loaded in the capacitor C.sub.1 is conducted into the capacitor C.sub.2, which is then loaded in turn. The above described loading of a capacitor and saturation of a saturable inductor occur succcessively in the first, second, and n-th stage of the LC circuit, while maintaining the energy of an input current wave and simultaneously successively compressing the pulse width. As a result, a high power pulse having a short pulse width is generated. The saturable inductors L.sub.1, L.sub.2, ------, L.sub.n comprise a saturable core.
A saturable core must, first, have a good saturation property, that is, a high squareness ratio and a low permeability at a saturation region of the hysteresis loop (hereinafter referred to as .mu.sat). The present inventors investigated the saturation property and concluded that when the ratio Br/B.sub.10 is at least 0.7, a good saturation property is obtained. Here, Br is the residual flux density, and B.sub.10 is the magnetic flux density at magnetizing a field of 10 Oe, as shown in FIG. 2. Since .mu.sat is proportional to the volume of the saturable core, and vice versa, when .mu.sat is small, the saturable core is advantageously small sized.
The theoretical maximum compression coefficient of the pulse width is proportional to (.mu.unsat/.mu.sat).sup.1/2, wherein .mu.unsat indicates the permeability at the unsaturated region of the hysteresis loop. Thus, the greater the difference between .mu.unsat and .mu.sat, the higher the theoretical maximum compression coefficient of pulse width, with the result that the number of stages of the LC circuit can be decreased and, thus, the magnetic switch can be made smaller.
Since a saturable core must, second, be energized or magnetized, as shown in FIG. 2 in such a manner that its magnetic flux density increases from -Br to Bs of the hysteresis loop, the .DELTA.Bs (.vertline.-Br.vertline.+Bs) must be great. The time required for increasing the magnetic flux density from -Br to Bs is referred to as the switching time.
A saturable core must, third, have a low power loss (watt loss) at a high frequency, since the saturable core is energized or magnetized under an alternating current having a frequency of approximately 10 kHz or more. The power loss is generally proportional to the thickness of the material and is influenced by its composition.
A saturable core must, fourth, be resistant to secular changes of magnetic properties.
Conventional saturable cores are made of ferrite, crystalline nickel-iron alloys, or other crystalline alloys. Recently, amorphous alloys, also referred to as "metallic glasses", "ferromagnetic amorphous metals", and the like depending on the technical field, have also attracted attention as materials for saturable cores. Amorphous alloys have high resistivities, therefore, low power loss, compared to the crystalline nickel-iron alloys and have high saturation inductions together with high squareness ratios.
C. H. Smith et al, in "Amorphous Metal Reactor Cores for Switching Applications". Proceedings of the 3rd International Power Conversion Conference, Munich (September 1981), disclosed several amorphous alloys pertinent to saturable cores, i.e., a 33 .mu.m thick Fe.sub.67 Co.sub.18 B.sub.14 Si.sub.1 sheet and a 30 .mu.m thick Fe.sub.81 B.sub.13.5 Si.sub.3.5 C.sub.2 sheet. M. Stockton et al in "Pulsed Power Switching Using Saturable Core Inductors", Journal of Applied Physics 53 (3) (March 1982), discloses single-turn saturable cores constructed of Fe.sub.67 Co.sub.18 B.sub.14 Si.sub.1 amorphous alloy for switching fast, high-power pulses. Such saturable cores, however, do not satisfy all of the four properties described above.
Carl H. Smith, in "Metallic Glasses for Magnetic Switches, IEEE Conference Record of 15th Power Modulator Symposium, June 14 to 16, 1982, Baltimore, Md., Pages 22 to 26", discloses the necessity of insulation to reduce short-circuiting and inter-laminar eddy current and also insulation methods, such as coating and insertion of a separate inter-laminar layer with margins.
Saturable cores are manufactured by winding an amorphous alloy sheet, e.g., in the form of a toroid. Inter-layer short-circuiting is likely to occur in the saturable core, since the magnetic flux density instantaneously changes and a high voltage which is proportional to that change is generated when a high frequency current is applied to the saturable core.
For a transformer core, a silicon steel sheet is high-temperature annealed and a glass film is formed on it during the annealing. An insulation film is then applied on the silicon steel sheet and baked. However, since the saturable core comprises a wound amorphous alloy sheet, and since amorphous alloy is much less thermally stable than silicon steel, the insulation film used for silicon steel sheet cannot be employed for the layer insulation of a wound amorphous alloy sheet.
The layer insulation of a wound amorphous alloy sheet is conventionally carried out by applying MgO or another insulating material. The method for applying the insulating material is not practical, however, since the edges of an amorphous alloy sheet are sharp and, thus, are not covered by the insulating material. Thus, short-circuiting is likely to occur between the edges of neighboring layers of an amorphous alloy sheet.
Layer insulation of a saturable core is also conventionally carried out by winding a polyimide or polyethyleneterephthalate film together with a amorphous alloy sheet, thereby inserting the film between the layers. Since polyimide or the like is not very heat resistant, however, it cannot withstand the heat treatment meant to improve the magnetic properties, especially .DELTA.Bs, of an amorphous alloy sheet, which treatment is carried out at a temperature below the crystallization temperature and ranges from 300.degree. C. to 500.degree. C. Therefore, the amorphous alloy sheet must first be heat treated and then wound together with a film of polyimide or the like to obtain the layer insulation.
Carl H. Smith also discloses 18 .mu.m thick iron based ribbons as saturable cores. Although 18 .mu.m thick iron-based ribbons have occasionally been produced, however, and they feature relatively low power loss, they are not totally satisfactory in the other three properties.